This invention relates to a metal vapor-trapping system which comprises cooling a gaseous mixture containing vapors of liquid metal, thereby trapping, said metal vapors by condensation and solidification.
Devices in common use for removing vapors of liquid metal from a gaseous mixture by cooling include a condensation type which traps metal vapors by condensation into a liquid form and a solidification type which traps metal vapors by solidification. The condensation type trapping device which simply consists in condensing metal vapors can be operated continuously for long hours by properly adjusting the liquefying rate through control of cooling temperature, but has the drawback that metal vapors can not be fully eliminated, namely, said trapping is carried out inefficiently. On the other hand, the solidification type trapping device can indeed attain a substantially 100% removal, but is accompanied with the shortcoming that the period of its continuous operation is short. The reason is that the solidification type trapping device is so designed as to cool metal vapors for solidification and deposit solidified metal on packing acting as a filter, and that the packing having a limited volume of deposition quickly has its meshes plugged with solid deposits of metal when a large amount of metal vapors is introduced into said device, failing to be used any longer.
As mentioned above, no satisfactory metal vapor-trapping device has been developed which attains said trapping at a high rate and can be operated continuously for long hours.
Such a metal vapor-trapping apparatus is applied, for example, in the case where it is desired to trap vapors of molten lithium carried into argon gas or vapors of molten aluminium brought into nitrogen gas.
Particularly in recent years, where detection is to be made of any failing fuel rod in a nuclear reactor using liquid metal, for example, molten sodium as a coolant, it is necessary to trap vapors of molten sodium carried into the inert gas, for example, argon gas enveloping active molten sodium. Detection of a failing fuel rod is carried out by sampling said inert gas enveloping the active molten sodium used as a coolant in a liquid metal coolant reactor to determine whether or not said inert argon gas contains fission products, for example, krypton (Kr) and xenon (Xe). The molten sodium used as a coolant is generally heated to about 500.degree. C in the nuclear reactor, so that the inert argon gas enveloping the molten sodium is unavoidably contaminated with its vapors. Therefore, sampling of such argon gas for detection of any failing fuel rod simultaneously collects the vapors of the molten sodium. However, said vapors are already activated and moreover display electric conductivity when condensed or solidified, thus exerting a harmful effect on a detecting device. It is therefore necessary to trap in advance the vapors of the molten sodium carried into the argon gas. This necessity also occurs with the cask-car of a fuel handling mechanism included in a nuclear reactor system. Namely, the vapors of molten sodium deposited on the surface of a spent fuel rod is carried into the argon gas enveloping said molten sodium. And the vapors of molten sodium are collected in a gas circulation system, giving rise to various harmful effects. Accordingly, the cask-car should be provided with a device for trapping the vapors of molten sodium carried into the argon gas.